Before proceeding to solder one has to apply some sort of insulation over the via(piece of tape,girl-friend's nail polish) so that the body of the switch doesn't touches the via.Since the switch is already soldered try raising it a bit(very carefully or you may damage the tracks) so that it doesn't touches the via.
BTW I've used jumpers (gain= 1X, 2X & 8X) to avoid this problem.
see #3147The Objective2 (O2) Headphone Amp DIY Project

Thanks! I'm on the process of fixing, but is having a hard time getting the switch off the board (so I can cut off the bottom left leg (closest to the switch head), correct?).

Also, one little question. The sound on my self built O2 seems to be a little muffled and less detailed compared to a stock O2 amp. Are there any suggestions of what may be wrong? Perhaps too much solder, or some components are not actually making contact, but instead making contact through the solders? Or perhaps because I cut the power jack because it won't fit, and the electricity might be flowing through the solder onto the board instead of directly onto the board?

I noticed something strange while testing my O2. With the amp switched off, putting a sine wave on the input produces a DC voltage at the output. The DC increases in proportion with the input voltage. Using a 1KHz 2.5Vp-p sine wave produces -1.53VDC with around 500 ohms output impedance. The strange thing is, this happens even with the volume at minimum.
Anyone got any clue what's going on here? Seems very strange that it's doing this with no power and volume at minimum.

I noticed something strange while testing my O2. With the amp switched off, putting a sine wave on the input produces a DC voltage at the output.

Atilla ran into that problem in Feb of last year, post #2012 in this thread. There are some more details in the posts after that one. I don't think the cause and resolution were ever found. My best guess was something in the NJM2068 chip was rectifying the incoming signal, even with no power to the chip, then sending that back out the power pins where C8 and C9 then charged up to some DC voltage, partially powering on the output NJM4556A chips.

With the power to the O2 off the mosfets would be off, which would cut that output power rail segment off from the power managment circuit and voltage regulators (load), leaving C8 and C9 as the defacto power rail filter caps for the resulting isolated power rail segment past the mosfets.

You have a good test setup there! I would be curious what the voltages at pin 4 and 8 (power pins) to ground are on the NJM2068 when it happens. Even better, it would be interesting to see a waveform to ground on those pins. I wouldn't be surprised if it is rectified 1kHz, being smoothed then by C8 and C9. Might also just be a single ended voltage being produced, with the v- pin as essentially ground and the rectified 1Khz coming out the V+ pin, or vice versa.

weslito - putting 1N4148 diodes on the NJM2068 power pins as below might be one fix if that turns out to be the problem. The diodes would block current from flowing in the reverse direction when the power rails are off. I picked silicon 1N4148's here instead of Schottky's since the reverse current leak is so much great with the Schottky's.

Just bending the power pins up on a sacraficial NJM2068 and tack soldering diodes to them would be a way to test it without having to cut any traces. Then the free ends of the diode lead stuff back down into the IC socket holes. I forgot that was how I tested the NJM4556 feedback resistor mod.

wiinippongamer also just let me know that a fix I came up with in January for power management turn-off transients he was getting while on batteries has worked after 6 weeks of testing. Just a 10K resistor soldered across the power rails at C8 and C9 to put a small load on that post-mosfet segment once the mosfets cut off. The 10K acts as a bleed resistor for those caps.

The theory was that the NJM4556A and NJM2068 chips are only specified down to about 2.5Vdc, so anything below that as the rails bleed down is kind of no man's land. I was thinking the chips may cut off their power draw hard below 2.5Vdc and leave the rails charged, without a bleed resistor, and that appears to be the case. Adding that 10K bleed load might also help with this problem.

Two things, is there an output jack that fits the pcb can accept TRRS plugs? I drives me nuts sometimes.

Any ideas to bring down the volume to attenuate the signal? With my current IEMs I can't imagine going over 12 o'clock on the volume pot. So I need about 0.1 gain / 10x attenuation. Guess I'll measure the pot before to determine the right value.

My goals: sustaining good quality (noise, distortion) > easily to do without to much cutting traces etc. >> can be neutralized by the gain switch.

There are two solutions I can think of.
- increase R7 and R3 to 9k, decrease C11 and C12 to a value that fits both low and high gain (22pf) and use R17 and R21 to switch 1k parallel to R14 and R20.
- put 9k between the pot and pin 1 and 7 of the NJM2068, use R17 and R21 to switch 1k parallel to the pot.

Namenlos, have you are already configured it for unity gain by removing R17 and R21? If so, the easiest way to achieve even lower gain would be swapping R7 and R3 for a higher value and resizing C11 and C12 appropriately, as you say.
You can calculate the voltage gain this would give you with:
R14/(R14+R7)
Using 9K will give you:
10000/(9000+10000) = 0.53

Quote:

Originally Posted by Namenlos

Two things, is there an output jack that fits the pcb can accept TRRS plugs? I drives me nuts sometimes.

Any ideas to bring down the volume to attenuate the signal? With my current IEMs I can't imagine going over 12 o'clock on the volume pot. So I need about 0.1 gain / 10x attenuation. Guess I'll measure the pot before to determine the right value.

My goals: sustaining good quality (noise, distortion) > easily to do without to much cutting traces etc. >> can be neutralized by the gain switch.

There are two solutions I can think of.
- increase R7 and R3 to 9k, decrease C11 and C12 to a value that fits both low and high gain (22pf) and use R17 and R21 to switch 1k parallel to R14 and R20.
- put 9k between the pot and pin 1 and 7 of the NJM2068, use R17 and R21 to switch 1k parallel to the pot.